The present invention relates to a method of controlling the moisture profile in paper web in a paper drying process and an apparatus practicing same, and more particularly to an improvement of the drying part in a paper making process in which synthetic fabrics are fixedly wound around the appointed position of the surface of a drying cylinder to utilize the void areas among yarns of said synthetic fabrics as insulating air layers, wet paper web being dried through said insulating air layers whereby the moisture profile in said paper web is equalized all over said paper web.
A paper making machine provided with a multi-cylinder type drying mechanism has been used as an apparatus of making various kinds of paper material. Such a paper making machine is provided with a drying mechanism, which consists of several types of drying cylinders, in order to dry wet paper web continuously fed thereto through a web forming process and a pressing process. Although such drying cylinders are constructed in such a manner that steam is introduced thereinto and wet paper web is contacted with metallic cylinders, which are heated to the appointed temperature by the steam, through dryer felt to dry the wet paper web, in general the wet paper web is not even in the moisture profile, the moisture profile showing considerable fluctuations along the machine direction as well as the cross machine direction. On the other hand, since the width of drying cylinders is usually designed so that it may be larger than the width of paper web, there are remained the ranges, where the drying cylinders are not brought into contact with the wet paper web, at both end parts of a circumferential surface of the drying cylinders. Therefore, the surface temperature of the drying cylinders near both end parts of the circumferential surface thereof is apt to be higher than that in the region where the drying cylinders are brought into contact with the wet paper web. Thus, an edge portion of the wet paper web passing by the vicinity of both end parts of such drying cylinders is overdried due to its contact with the drying cylinders heated to temperatures higher than the appointed temperature. In the drying part, the wet paper web is dried not only by heat transmitted directly from the drying cylinders but also by the displacement of wet air to fresh dried air through the dryer felt in the pockets (a region defined by said felt and said wet paper web). That is to say, water evaporated from the wet paper web is discharged out of the pocket through the felt and dry air comes in the pocket from the outside thereof to promote the evaporation of water. This effect is called in general the natural ventilation effect of the drying pocket. The permeability of the dryer felt is designed so that the natural ventilation effect may be carried out in a well balanced condition. It was, however, unavoidable that the drying rate is apt to increase owing to the influences of cylinder end, an air current rising directly from the floor and the like, in the vicinity of the both end parts of the drying cylinders. In addition, the degree of overdrying is apt to fluctuate between the both end parts of the drying cylinders in general owing to the different constructions of the passages of heating medium formed in the drying cylinders. Although both end parts of wet paper web were very frequently overdried owing to the above mentioned various kinds of cause, sometimes for example the central part of wet paper web showed drying streaks according to circumstances, whereby various kinds of defect such as curling and others were produced to debase the quality of the final products. A large number of technical means have been practiced or proposed in order to obviate such troubles. For example, a method of controlling the quantity of water evaporated from wet paper web by adjusting the permeability of dryer felt in the cross machine direction thereof by means of the changes of weaving density, the treatment of an edge part with resins and the like whereby the volume of air coming in and going out of a pocket is changed; a method of reducing the surface temperature of both end parts of drying cylinders by changing the arrangement of steam supplying routes in said drying cylinders and opening drainage passages on the internal surfaces of both end parts of said drying cylinders; a method of adjusting temperature changes on the surface of drying cylinders by dividing the inside of said drying cylinders into a plurality of subdivision along the axial line thereof and providing steam supplying passages at different intervals in different subdivisions; various kinds of pocket ventilation apparatus and the like have been practiced or proposed. However, according to these means, it has been difficult to achieve the proper maintenance of an apparatus in spite of much cost and labour required for processing dryer felts or drying cylinders and the like. In addition, it has been difficult for these means to be promptly adapted as the moisture profile adjusting means for paper web of which moisture profile is changed time to time or in correspondence to the kind of the paper web to be dried.
Another method of preventing edge portions of a wet paper web from being overdried by adhering glass fiber cloths coated with fluorine resin to both end parts of the circumferential surface of drying cylinders to form an insulating layer also has been practiced in U.S. Pat. No. 4,192,080. However, according to this method, since the meshes of glass fiber cloths are substantially closed by fluorine resin coated on said glass fiber cloths as a coating agent and it is necessary to increase the thickness of resin layer coated on said glass fiber cloths to reinforce said glass fiber cloths owing to little resistance of glass fibers to abrasion, highly insulating air can not be held within said meshes and the above described function of preventing said edge portion from being overdried is remarkably spoiled.